DESCRIPTION: (Applicant's Abstract) Drug misuse by injecting is a global problem, which is increasing in incidence in many countries, and which is associated with increased morbidity and mortality particularly in young people. Injecting drug use is also a major risk activity for the spread for HIV infection. This cohort will study the relationship between microglial activation and neuronal damage in a clinically well characterized cohort of drug users in Edinburgh, Scotland, who have a high prevalence of chaotic and injecting drug use, and of HIV infection. Their lifetime drug history, and premorbid and comorbid neuropsychological and general health status is well documented. Despite early identification of the HIV epidemic in this group, and a programme of methadone substitution, the mortality rate has been high, both from drug related causes and from associated HIV and hepatitis C infections. The cohort has been characterized by a very high prevalence of HIV encephalitis [59b/o] when they progressed to AIDS. This study will include patients with pure HIV encephalitis who did, or did not, use drugs, drug users who were HIV negative and controls who were neither HIV infected nor used drugs. Immunocytochernical methods will be applied to detect the neuroimmune status particularly in the frontal lobe, basal ganglia, temporal hippocampus and brain stem nuclei, which are areas of the brain most concerned with cognitive function or in which we have observed evidence of neurodegeneration. We expect to demonstrate that microglial activation, and evidence of neurodegeneration, are present in both drug users and in patients with HIV infection, and that the effects are cumulative in drug users with HIV encephalitis. We expect that this study may reveal whether microglial activation and cytokine production precede, or are the result of, neuronal degeneration in drug users. We will relate these findings to evidence of synaptic damage in order to explore further the basis of cognitive impairment. Morbidity problems associated with drug misuse are a significant drain on the health budget of many nations, quite apart from the more general impact on society. Elucidation of the interaction between drugs and HIV infection of the CNS could lead to more focused strategies to prevent cognitive decline in drug users, particularly those who are also HIV infected.